refactor(config): auto-update agent instructions, never create new files (#1027)

## Summary

- `vp config` no longer creates new agent files or prompts for agent selection
- Silently updates existing files that contain Vite+ markers (`<!--VITE PLUS START-->`) when content is outdated
- Same behavior for `prepare` lifecycle, interactive, and non-interactive modes
- Skipped with `--hooks-only`
- Delete `config/agent.ts` and its tests (superseded by shared `utils/agent.ts`)

## Behavior

| Scenario | Action |
|----------|--------|
| No agent files | No writes |
| Agent files exist, no Vite+ markers | No writes |
| Vite+ markers, content up to date | No writes |
| Vite+ markers, content outdated | Update marked section silently |
| `--hooks-only` | Skip agent update |

## Test plan

- [x] `pnpm -F vite-plus snap-test-global command-config` — all 9 tests pass
- [x] `pnpm -F vite-plus snap-test` — all tests pass
- [x] New snap test: `command-config-update-agents` — verifies outdated markers are updated, user content preserved
- [x] New snap test: `command-config-no-agent-writes` — verifies no writes when no markers, no new files created

Author: fengmk2

packages/cli/snap-tests-global/command-config-no-agent-writes/CLAUDE.md

@@ -0,0 +1,3 @@
+# My Custom Claude Instructions
+
+Do not modify this file.

packages/cli/snap-tests-global/command-config-no-agent-writes/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "command-config-no-agent-writes"
+}

packages/cli/snap-tests-global/command-config-no-agent-writes/snap.txt

@@ -0,0 +1,9 @@
+> git init
+> vp config
+> cat CLAUDE.md # should be unchanged
+# My Custom Claude Instructions
+
+Do not modify this file.
+
+> test -f AGENTS.md && echo 'AGENTS.md exists' || echo 'AGENTS.md not created' # should not exist
+AGENTS.md not created

packages/cli/snap-tests-global/command-config-no-agent-writes/steps.json

@@ -0,0 +1,8 @@
+{
+  "commands": [
+    { "command": "git init", "ignoreOutput": true },
+    "vp config",
+    "cat CLAUDE.md # should be unchanged",
+    "test -f AGENTS.md && echo 'AGENTS.md exists' || echo 'AGENTS.md not created' # should not exist"
+  ]
+}

packages/cli/snap-tests-global/command-config-update-agents/AGENTS.md

@@ -0,0 +1,11 @@
+# My Project
+
+Custom instructions here.
+
+<!--VITE PLUS START-->
+
+OUTDATED CONTENT THAT SHOULD BE REPLACED
+
+<!--VITE PLUS END-->
+
+More custom content below.

packages/cli/snap-tests-global/command-config-update-agents/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "command-config-update-agents"
+}

packages/cli/snap-tests-global/command-config-update-agents/snap.txt

@@ -0,0 +1,16 @@
+> git init
+> vp config # should auto-update agent instructions
+> head -5 AGENTS.md # verify user content preserved
+# My Project
+
+Custom instructions here.
+
+<!--VITE PLUS START-->
+
+> tail -3 AGENTS.md # verify user content preserved
+<!--VITE PLUS END-->
+
+More custom content below.
+
+> grep -q 'OUTDATED CONTENT' AGENTS.md && echo 'ERROR: outdated content still present' || echo 'outdated content replaced' # verify old content gone
+outdated content replaced

packages/cli/snap-tests-global/command-config-update-agents/steps.json

@@ -0,0 +1,9 @@
+{
+  "commands": [
+    { "command": "git init", "ignoreOutput": true },
+    "vp config # should auto-update agent instructions",
+    "head -5 AGENTS.md # verify user content preserved",
+    "tail -3 AGENTS.md # verify user content preserved",
+    "grep -q 'OUTDATED CONTENT' AGENTS.md && echo 'ERROR: outdated content still present' || echo 'outdated content replaced' # verify old content gone"
+  ]
+}

packages/cli/snap-tests/command-run-with-vp-config/snap.txt

@@ -2,21 +2,6 @@
 $ vp config ⊘ cache disabled
 .git can't be found
 
-Created AGENTS.md with Vite+ instructions
-◇  Add this MCP server config to your agent ─╮
-                                              
-   {                                          
-     "vite-plus": {                           
-       "command": "npx",                      
-       "args": [                              
-         "vp",                                
-         "mcp"                                
-       ]                                      
-     }                                        
-   }                                          
-                                              
-╰────────────────────────────────────────────╯
-
 
 [2]> vp run bar # should throw error
 $ vp not-exist-command ⊘ cache disabled